Synthetic metal oxide single-walled nanotubes (SWNTs) can be expected to possess a number of interesting and unique properties, and are of interest in a number of applications such as molecular separation, chemical sensing, catalysis, and photonics. Although the number of such SWNT materials is not yet extensive, one attractive aspect of such materials is that they can be fabricated by liquid-phase synthesis under moderate conditions.
More specifically, a particular metal oxide (aluminosilicate) SWNT, which is a synthetic version of the nanotubular mineral imogolite, has attracted substantial interest in recent years. This SWNT consists of a tubular aluminum(III) hydroxide layer on the outer surface with pendant silanol groups on the inner surface (FIG. 1a). Additionally, the proposed mechanisms of single-walled metal oxide nanotube growth allow several possible ways of controlling the monodispersity, composition, and structure of the nanotube product.
A long-standing issue in nanotube science and engineering is the modification (functionalization) of SWNT surfaces. Modification of the inner or outer surface of the nanotube with functional entities would greatly expand both their properties and their applications. For example, an outer-surface modification could increase their compatibility with a solid- or liquid-phase materials, whereas an inner-surface modification would be advantageous for shape/size-selective separations and catalysis.
Diverse approaches for outer-surface modification of carbon nanotubes have been reported. However, the modification of the inner wall of carbon nanotubes is much more difficult, due to the low reactivity of the inner wall, as well as due to the mesoporous and microporous silicas. The capability to control the chemistry of the inner surface of the aluminosilicate SWNTs thus has significant implications for nanotube science and engineering.
There have been several reports on the outer-surface modification of single-walled aluminosilicate nanotubes. However—as in the case of carbon nanotubes—inner wall modification is much more difficult, and no convincing results suggesting inner wall functionalization have been published. We hypothesize that the extraordinarily high surface silanol density of the inner wall (˜9.1 —OH/nm2) makes the material highly hydrophilic at ambient conditions (FIG. 1b), and that the strong binding of water molecules to the inner wall hinders functionalization of the inner surface.
Therefore, a comprehensive knowledge of dehydration and subsequent dehydroxylation phenomena (due to condensation of hydroxyls) on the SWNT surface is critical for accessing reactive surface sites and for creating a new class of inner-wall-functionalized SWNT materials.
Previous studies have reported on the dehydration, dehydroxylation and pore collapse in these SWNTs via solid-state NMR, infrared spectroscopy (IR), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). However, such studies reach varying conclusions on many issues, for instance the heat treatment temperature required to completely dehydrate the inner wall, and the temperature required for nanotube collapse. Previous studies have proposed dehydroxylation models in the absence of definitive supporting evidence. Furthermore, a quantitative study on the pore volume of the SWNT in different dehydrated and dehydroxylation conditions—a prerequisite for inner-wall modification studies—is still lacking.
Herein we report a systematic qualitative and quantitative investigation of dehydration and dehydroxylation phenomena in aluminosilicate SWNTs over a wide temperature range of 25-450° C. The structure and composition of the SWNT is assessed by a combination of techniques including in situ XRD, FTIR, NMR, TGA-MS, and N2 physisorption. Based upon our results, a quantitative model is proposed for the dehydration and dehydroxylation phenomena occurring in the SWNT upon heat treatment. Furthermore, a unique rehydroxylation phenomenon that occurs in the dehydrated SWNT upon re-exposure to water is elucidated. As a result, this study leads to the preparation of a range of well-characterized heat-treated materials amenable to inner-wall surface functionalization.